Subsurface safety valves, such as a tubing retrievable safety valves, deploy on production tubing in a producing well. The safety valves can selectively seal fluid flow through the production tubing if a failure or hazardous condition occurs at the well surface. In this way, safety valves can minimize the loss of reservoir resources or production equipment resulting from catastrophic subsurface events.
A conventional safety valve uses a flapper to close off flow through the valve. The flapper, which is normally closed, can be opened when hydraulic pressure applied to a hydraulic piston move a flow tube against the bias of a spring in the valve. When the flow tube moves, it pivots the flapper valve open, allowing flow through the safety valve.
From the surface, a control line supplies the hydraulic pressure to operate the valve. The control line extends from a surface controlled emergency closure system, through the wellhead, and to the safety valve. As long as hydraulic pressure PC is applied through the control line, the valve can remain in the opened position, but removal of control line pressure returns the valve to its normally closed position. The hydrostatic or “head” pressures PH from the column of fluid in the control line can directly limit the setting depth and operational characteristics of the safety valve in such a system.
Historically, additional load from stronger power springs has been used to offset the hydrostatic pressure of the control line. However, safety valves have limited space available to accommodate a larger spring. In fact, the active control line hydrostatic pressure PH can be so significant in some applications that a spring may not be able to overcome the hydrostatic pressure and the valve's flapper cannot close, assuming the wellbore pressure is zero.
To compensate for the control line's hydrostatic pressure PH, a gas (nitrogen) charge can be stored in the safety valve to counteract the hydrostatic pressure. Unfortunately, using a gas charge in the valve presents problems with leakage of the gas, which can cause the valve to fail in the open position. In addition, once the charge is spent in a fail-safe operation, operators must do a substantial amount of work to replace the valve.
In contrast to a gas charge, safety valves have been developed that use a magnetically driven device on the valve. The magnetic device allows the hydraulics to reside outside the wellbore and may use annulus pressure to offset the hydrostatic pressure of the control line so that the safety valve can be set at greater depths. Unfortunately, using such an arrangement may be undesirable in some applications.
In yet another solution, a second “balance” control line has been used with a deep-set safety valve to negate the effect of hydrostatic pressure PH from the active control line. In these existing balance line valves, the second balance line acts on the valve's piston against the pressure from the active control line to balance the hydrostatic pressure PH from the active control line Therefore, because the underside of the piston is in fluid communication with the balance line, the piston is no longer in fluid communication with the tubing. Accordingly, any beneficial effect produced by the tubing pressure PT in operating this type of deep-set safety valve is not utilized.
A different type of balance line arrangement shown in FIG. 1 is disclosed in U.S. Pat. No. 7,392,849, which is assigned to the Assignee of the present disclosure and is incorporated herein in its entirety. Production tubing 20 has a deep-set safety valve 50 for controlling the flow of fluid in the production tubing 20. In this example, the wellbore 10 has been lined with casing 12 with perforations 16 for communicating with the surrounding formation 18. The production tubing 20 with the safety valve 50 deploys in the wellbore 10 to a predetermined depth. Produced fluid flows into the production tubing 20 through a sliding sleeve or other type of device. Traveling up the tubing 20, the produced fluid flows up through the safety valve 50, through a surface valve 25, and into a flow line 22.
As is known, the flow of the produced fluid can be stopped at any time during production by switching the safety valve 50 from an open condition to a closed condition. To that end, a hydraulic system having a pump 30 draws hydraulic fluid from a reservoir 35 and communicates with the safety valve 50 via a first control line 40A. When actuated, the pump 30 exerts a control pressure PC through the control line 40A to the safety valve 50.
Due to vertical height of the control line 40A, a hydrostatic pressure PH also exerts on the valve 50 through the control line 40A. For this reason, a balance line 40B also extends to the valve 50 and provides fluid communication between the reservoir 35 and the valve 50. Because the balance line 40B has the same column of fluid as the control line 40A, the outlet of the balance line 40B connected to the valve 50 has the same hydrostatic pressure PH as the control line 40A.
Internally, components of the safety valve 50 are exposed to control pressure PC from the control line 40A and the offsetting hydrostatic pressure PH from the balance line 40B. Yet, the components are also exposed to tubing pressure PT in the well during operation, which can be beneficial. As briefly illustrated in FIGS. 2A-2B, the deep-set safety valve 50 uses the hydraulic pressures from the two control lines (40A-B) so the valve 50 can be set at greater depths downhole. The valve 50 as illustrated in FIGS. 2A and 2B has first and second actuators 60A-B. The first actuator 60A has an active piston 62A coupled to a flow tube 54. Control pressure from the primary control line (40A) moves the control piston 62A and the flow tube 54 against the bias of a spring 56 to open the valve's flapper (not shown). The second actuator 60B has a balance piston 62B that can intermittently engage the flow tube 54 during operation.
In FIG. 2A, the valve 50 is in a closed condition where the balance piston 62B is idle in which case the tubing pressure PT is greater than the hydrostatic pressure PH. By contrast, the valve 50 is in an opened condition in FIG. 2B. As shown in FIG. 2A, if the tubing pressure PT is substantial, then force from this tubing pressure PT and from the spring 56 exerts on the control piston 62A and tends to close the valve 50. Since the tubing pressure PT is greater than PH in FIG. 2A, however, the balance piston 52B is idle as it exerts no force on the flow tube 54 because a net downward force exerted by the tubing pressure PT keeps the balance piston 62B resting on a shoulder 57.
As shown in FIG. 2B, if the hydrostatic pressure PH is substantial, a force exerts on the control piston 62A and tends to open the valve 50. Likewise, control pressure PC from the control line (40A) exerts on the control piston 62A and tends to open the valve 50. Yet, the hydrostatic pressure PH exerts an opposing force on the balance piston 62B, thereby tending to close the valve 50. Additionally, the tubing pressure PT exerts an opposing force on the balance piston 62B; however, this force does not tend to open the valve 50 because the balance piston 62B is structurally isolated from the flow tube 54 (and the spring 56) by interaction of a block 55 with the shoulder 57 of the chamber housing. Thus, if the control pressure PC is reduced in FIG. 2B, the valve 50 will revert to the closed condition shown in FIG. 2A.
Although existing safety valves for deep-set applications may be effective, operators are continually seeking improved hydraulic control systems for deep-set applications that can avoid failures and mitigate other problems. The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.